TEMPERATURE OF THE TERRESTRIAL STRATA. 425 



which the history of discoveries reveals in a thousand 

 places : " When a thing may be in two different ways, it 

 is almost always that which appears at first the least 

 natural." 



"Whatever importance these reflections may possess, I 

 hasten to add that, instead of the arguments of his prede- 

 cessors, which have no real value, Fourier has substituted 

 proofs, demonstrations ; and we know what meaning such 

 terms convey to the Academy of Sciences. 



In all places of the earth, as soon as we descend to 

 a certain depth, the thermometer no longer experiences 

 either diurnal or annual variation. It marks the same 

 degree, and the same fraction of a degree, from day to 

 day, and from year to year. Such is the fact : what says 

 theory ? 



Let us suppose, for a moment, that the earth has con- 

 stantly received all its heat from the sun. Descend into 

 its mass to a sufficient depth, and you wiil find, with 

 Fourier, by the aid of calculation, a constant temperature 

 for each day of the year. You will recognize further, 

 that this solar temperature of the inferior strata varies 

 from one climate to another ; that in each country, finally, 

 it ought to be always the same, so long as we do not de- 

 scend to depths which are too great relatively to the 

 earth's radius. 



Well, the phenomena of nature stand in manifest con- 

 tradiction to this result. The observations made in a 

 multitude of mines, observations of the temperature of 

 hot springs coming from different depths, have all given 

 an increase of one degree of the centigrade for every 

 twenty or thirty metres of depth. Thus, there was some 

 inaccuracy in the hypothesis which we were discussing 

 upon the footsteps of our colleague. It is not true that 



